When performing machine processing over the entire surface of a workpiece or around the entire external circumference thereof, a clamp device of the type described above (a so-called hole clamp) is employed, since it is not possible to employ a clamp device of the type that presses from above with a pressing tool on an edge portion of the workpiece. With such a clamp device, a seating surface or surfaces on which the workpiece is to be seated is formed at an upper end portion of a clamp main body that is shaped as a block, so as to surround a grip member; and when the workpiece is to be loaded: the workpiece is mounted on this seating surface and supported thereon; a grip claw portion of the grip member and a clamp rod having a taper shaft portion inserted through the grip member are inserted into a hole in the workpiece; and, after having made the grip claw portions expand in diameter with a taper shaft portion by pulling the clamp rod towards the seating surface, so that the grip claw portions engage with the inner circumferential surface of the hole, the workpiece is fixed against the seating surface by pulling this grip member further towards the seating surface with the clamp rod.
In Patent Documents #1 and #2, clamp devices of the abovementioned type are disclosed. In particular, with the clamp device described in Patent Document #1, in order to cope with variations in the position of the hole in the workpiece (i.e. manufacturing errors), the clamp rod having a taper shaft portion and a grip member, are adapted to be movable in directions orthogonal to the axis, so that thereby it is possible to perform reliable clamping, even if the position of the hole in the workpiece is deviated more or less from its proper original position.
Now, as with the clamp devices of Patent Documents #1 and #2, this type of clamp device has a grip member, a clamp rod, a hydraulic cylinder for clamping, and a clamp main body to which these elements are installed; and, normally, the clamping hydraulic cylinder is provided at a position in the vicinity of the center of the lower portion of the interior of the clamp main body which is shaped as a block, and the grip member and the clamp rod are arranged above the clamping hydraulic cylinder, with the grip member, the clamp rod, and the clamping hydraulic cylinder having the same common axis. Moreover, with the seating surface also being formed at the upper end of the clamp main body around the external circumference of the grip member, a clamping hydraulic cylinder of sufficient capacity may be employed.    Patent Document #1; Japanese Pat. No. 3,550,010 Publication    Patent Document #2; German Pat. No. 4,020,981 Publication